Self-orienting casters constituted by a single wheel or by a pair of wheels which are mounted so that they can rotate on a horizontal shaft are known.
A vertical cylindrical recess is provided in the supporting structure, is open upward, is axially offset with respect to the shaft, and acts as a rotation seat for a vertical pivot for the self-orientation (pivoting) of the wheel, which is designed to be inserted and retained in a receptacle of the piece of furniture to which the wheel is to be fitted.
Self-orienting casters are also known in which the wheels can rotate on a tubular shaft which protrudes from opposite sides of the central supporting structure. Casters of this type, also known as hubless casters, are disclosed in U.S. Pat. No. 978,161, in US publication no. 2002/01758539 and in GB-967,915, in German publication no. 101 28 099, and in Italian utility model no. 238,060. CH-477,999, GB-967,915 and US publication no. 2002/1178539 disclose solutions in which the wheels and the tubular shaft have mutually opposite tracks for the rolling of balls or rollers for rotary support of the wheels.
In known casters of the so-called twin type, i.e., casters comprising two wheels mounted at the opposite ends of a single shaft which is supported at the center, the shaft supporting structure is symmetrical with respect to the central plane of the caster and load stresses are equally divided between the two wheels. This symmetry, taking into account the fact that the supporting structure of the casters is provided by molding plastic material, ensures considerable advantages, both because it facilitates the manufacture of simpler molds and stronger casters and because it allows to obtain products which are aesthetically appreciable for being fitted to items of furniture (chairs, tables, et cetera). This applies in particular to the case of so-called hubless casters.
However, the structure of the support of hubless casters is not suitable when the caster comprises a single wheel. In this case, the supporting structure assumes an asymmetric configuration, which when stressed undergoes deformations and causes problems in functionality and ease of rolling, in addition to the fact that the assembly of the wheel is rather laborious.